New Holocene formal subdivision – application for the Russian Arctic

Budantseva Nadine Arkad'evna PhD in Geography Senior Scientific Reserach, the department of Geography, M. V. Lomonosov Moscow State University  119992, Russia, g. Moscow, ul. Leninskie Gory, 1, stroenie 19 nadin.budanceva@mail.ru Other publications by this author

Introduction

The problem of studying changes in natural conditions that occurred in past epochs is relevant because only by knowing these ancient processes, one can come closer to understanding the modern mechanism of natural phenomena and give a reasonable forecast of their further development. Against the general background of rhythmic and directional changes of the Quaternary period, the Holocene, the modern stage of the Earth's development, acts as an interglacial epoch, while the increasing strength of the anthropogenic factor gives great specificity to this time. Despite the relatively short duration of the Holocene – about 12 thousand years – this time was marked by significant changes in the natural environment, as convincingly evidenced by numerous geological, paleogeographic, geomorphological and many other data. The variability of climatic conditions in recent decades has predetermined the priority of studying changes in the Earth's climate in the past, as well as identifying their causes, patterns and consequences. The state of the cryolithozone plays one of the key roles both in the state of the Earth's climate system as a whole and for individual regions of the cryolithozone. The use of cryolithogenic natural archives of the Holocene – syngenetic re-vein ice – makes it possible to carry out paleogeographic reconstructions of a high degree of reliability and detail.

 The previous paradigm of slow, gradual climate change ^[1] at the beginning of the 21st century was replaced by the paradigm of events in which climate changes occur abruptly over short periods of time equal to centuries or less, against the background of relatively long periods of quasi-stable state. Similar changes that occurred during the winter season are well recorded using isotopic analysis of re-vein ice, since the resolution obtained during the study of ice and the scale of rapid climatic changes coincide. The relevance of the study of Holocene re-vein ice is also determined by the fact that they are the only reliable source of winter paleotemperature information for this territory. For the European North, due to the very limited finds of Holocene re-vein ice, obtaining winter paleoclimatic information becomes an important task.

According to the decision of the International Stratigraphic Commission in 2018, the Holocene received a new three-term division ^{[2, 3]}. The main objective of the study is to consider the criteria for the new division of the Holocene and compare the new periodization with the schemes previously proposed by Blitt–Sernander and N.A.Khotinsky, as well as the Holocene division scheme for the Arctic proposed by Yu.K.Vasilchuk.

The schemes of division of the Holocene by Blitt-Sernander and N.A.Khotinsky for northern Eurasia and Yu.K.Vasilchuk for the Russian Arctic: the main criteria for the allocation of periods

Speaking about the development of Holocene periodization schemes, it should be noted that they were created to establish a chronological connection between the zonal boundaries of regional diagrams based on radiocarbon, paleobotanical, stratigraphic and other data. The variant of the Blitt-Sernander scheme used in most of the temperate zone regions of both the northern and southern hemispheres can be called the earliest standard of Holocene periodization. This scheme includes 6 periods – Arctic (the oldest), subarctic, boreal, Atlantic, subboreal and subatlantic (the youngest), the criterion for dividing into periods in this scheme was a combination of two climatic parameters – heat and humidity. This scheme, originally created at the beginning of the XX century. for a relatively narrow region – Scandinavia – it subsequently spread widely and is used by many researchers up to the present day. The widespread use of this scheme, most likely, lies in the fact that it successfully reflected the periodization of the history of vegetation and climate of vast territories in the Holocene and made it possible to link events that took place at that time on land and in marine basins. Detailed radiocarbon dating (33 datings) of the 6th peat bog in the south of Sweden, in Ageredsmoss, and the pollen diagram obtained from it allowed T. Nilson ^[4] to chronologically link the Holocene periods and determine their age limits, while introducing his own terminology for the designation of the most ancient periods. The lower boundary of the Holocene and the oldest period, named by T. Nilson “pre-Boreal”, is dated to 10200 years, in northwestern and central Europe it is the period of domination of birch and pine forests and the predominance of cool and dry climate. This is followed by the Boreal period, the lower boundary of which is dated to 9500 years – this is the period of climate warming and the appearance of hazel, mixed and oak forests. The Atlantic period (the lower limit is 8000 years) is characterized by the maximum development of oak forests and corresponds to the climatic optimum of the Holocene. The subboreal period (the lower limit is 5000 years) is a period of relative cooling and a decrease in the participation of broad–leaved species in the composition of mixed forests. The sub–Atlantic period (the lower limit of 2100 years) is the time of increasing climate humidity and the spread of moisture-loving tree species (beech, hornbeam).            

Subsequently, N.A.Khotinsky ^[5] made adjustments to this scheme for northern Eurasia, shifting the time frames of some periods and proposed that the lower boundary of the Holocene is dated at 10500-10300 years, the preboreal-boreal – 9500-9000 years, the boreal-Atlantic – about 8000 years, the Atlantic-subboreal 5000-4500 years and the subboreal-subatlantic – in 2500-2200 years. These Holocene periodization schemes caused critical comments from some researchers who did not agree with the separation of, for example, two xerotherms, however, as a rule, no one objected to the main trend identified by the Blitt-Sernander scheme, which shows the transition from cold conditions at the end of the last glaciation to the post-glacial climatic optimum and subsequent cooling.

For the Arctic regions of the Russian cryolithozone, Yu.K.Vasilchuk in 1982 ^[6] proposed a three–term division of the Holocene, within which three stages were distinguished - pre-optimal, optimal and post-optimal. The time limits of the Holocene optimum were determined from 9 to 4.5 thousand years ago, it was considered as a period of increasing average annual air temperatures and a significant improvement in vegetation conditions, which resulted in the advancement of woody vegetation far into the zone of modern tundra, the intensification of thermokarst processes and the formation of peat bogs within the emerging thermokarst basins. At the same time, winter conditions during the optimum period were not milder than modern ones, and in some periods of winter they were colder, which contributed to the intensive growth of syngenetic re-vein ice.

A new division of the Holocene, adopted in 2018 by the International Commission on Stratigraphy: criteria for defining boundaries and global climate events

The new modern three-membered division of the Holocene was preceded by the refinement of the lower boundary of the Holocene, performed on the basis of the analysis of the isotope curves of Greenland and taking into account all the most ancient Holocene formations available, and especially the procedure for calibration of radiocarbon dates. In 2008, the International Commission on Stratigraphy (IUGS) established the boundary between the Holocene and the Neo-Pleistocene at the turn of about 11700 calibrated years ago. In 2018, the Holocene was divided by IUGS into three tiers (Fig. 1): Greenlandic (from 11700 to 8200 cal. l.n.), North Grippian (from 8200 to 4200 cal. l. n.) and Meghalayan (beginning of 4200 cal. l. n.).

Fig. 1. New division of the Holocene, according to the decision of the International Stratigraphic Commission in 2018 and comparison with the Blitt-Sernander scheme.

The new division of the Holocene was based on a natural, usually short-term episode recorded in climatic proxy data (i.e., changes in the oxygen isotopic composition in glacial cores and speleothems). Two such proxy events were used for global correlation of the boundaries of the Greenland/North Grippian and North Grippian/Meghalayan periods ^{[2, 3]}. The lower boundary of the Greenland period as the lower boundary of the Holocene was isolated in the NorthGRIP2 glacial core drilled on the central Greenland ice sheet (75.10°S, 42.32°W). This boundary corresponds to the first signs of climate warming at the end of the Late Dryas, dated at 11,703±99 years ago ^[7]. The lower boundary of the Northgripian period was determined in the Greenland ice core NorthGRIP1 (75.10 ° s.w., 42.32 ° w.d.) as a distinct signal of climate cooling, dated at 8,236 ± 47 years ago, which was caused by the discharge of water from glacial lakes into the Northern Atlantic Ocean after the retreat of the Laurentian ice sheet. It is believed that these discharges disrupted the existing model of currents in the ocean, which led to global cooling. Cold snap 8.2 thousand cal. years ago, it was recorded on the isotopic curve of the Greenland ice core in the form of a single distinct negative shift in the values of ?18 ^O, subsequently this event was recorded in many other paleoarchives ^[8].

The lower boundary of the Meghalayan period is dated to 4250 years ago by a change in the isotopic composition of oxygen speleothem in the cave of Maumluh (25°15’44”s.w., 91°42’54” w.d.) in the state of Meghalaya, Northeastern India. The 7 km long cave, located on the southern edge of the Meghalaya plateau, was formed on the contact of dolomite and sandstone of Eocene age. The location of the cave on the elevated southern edge (1300 m above sea level) of the plateau, between the Bay of Bengal and the Himalayas and the Tibetan Plateau, makes this area sensitive to various processes that determine monsoon activity in Asia. About 2 km northeast of the cave is the town of Cherrapunji, one of the rainiest places on Earth, where more than 11,000 mm of precipitation falls annually. The isotope-oxygen record on speleothems binds 4.2 thousand cal to the event. years ago, with a noticeable weakening of the Asian summer monsoon, which lasted about 200 years. It is believed that this event is recorded in many regions of the world, especially in the middle and low latitudes, and in general it was characterized by dry and cool climatic conditions. In the northern high-latitude regions during this period, glacial conditions are pronounced with a noticeable advance of glaciers ^{[2, 3]}.

It should be noted that in publications after 2018, the new three-term division of the Holocene is already actively used, since many proxy data (in addition to glacial cores) reflect the events of 8.2 and 4.2 thousand years, which are the boundaries of periods. Despite the fact that some modern researchers (for example, ^{[9, 10]}) in their publications use the scheme of division of the Holocene into early, middle and late, the age boundaries of these periods correspond to the boundaries in the new division approved by the International Commission on Stratigraphy (11.7, 8.2 and 4.2 thousand cal. years ago), and the identified local climatic trends during these periods generally correspond to global ones.

A new three-term division of the Holocene was used in the study of Holocene peatlands in the Appalachian Mountains in the eastern United States ^{[11, 12]}. At the same time, all three periods of the Holocene are fixed according to pronounced climatic trends. The Greenland period of the early Holocene was characterized in the eastern United States as a cool and dry period, which ended with a pronounced cooling of about 8.2 thousand cal. years ago. During the Northern Grippian period, an increase in the average annual air temperature was observed in the eastern part of the USA, culminating in the climatic optimum of the mid-Holocene. The climatic event of the beginning of the Meghalayan period is about 4.25 thousand cal. years ago, there was a large-scale and severe drought in the middle part of the country, in the east of the USA, the Meghalayan period as a whole was characterized by the predominance of humid climatic conditions. The peat bog stratigraphy and detailed radiocarbon dating made it possible to compare the climatic trends in the Holocene and identify the correspondence to the main events of the three periods. It is shown that during the Greenland period, the rate of peat accumulation decreased compared to the rate in the preceding late Dryas, and an even more noticeable decrease in the rate of peat accumulation was recorded during the North Grippian period. The revealed very low rate of peat accumulation during the climatic optimum, according to the authors, is the result of an increase in the rate of decomposition of peat due to a decrease in the groundwater level, as a result of climate change from cool and humid, prevailing in the Greenland period, to warm and dry in the Northern Grippian period. The fastest accumulation of peat is dated to the beginning of the Meghalayan period, during which wet conditions prevailed ^{[11, 12]}.

The boundary of the Greenland and North Grippian period (an event of 8.2 thousand cal. years) is recorded in a high-resolution record for bottom sediments in the northern part of the Ionian Sea (eastern part of the Mediterranean Sea). The cold phase, dated 9-8.2 thousand years ago, is determined by the inflow of colder waters, which caused mixing of the stratified water column and deep-sea waters with a low oxygen content ^[13]. Event 8.2 thousand cal. years ago, it was noted by the isotopic composition of foraminifera and bottom sediments in the Singapore area and records a decrease in the amount of precipitation between 8.14 and 7.96 thousand cal. years ago. These data show that in the western tropical Indo-Pacific region, there was a decrease in the convective activity of oceanic waters, at the same time, the manifestations of this event lagged behind the pronounced cooling in the North Atlantic and synchronous droughts in the Asian and Indian monsoon regions by about 100 years, which may be due to the propagation of the signal from the north to south along the oceanic route, which operates on a scale of hundreds of years ^[14].

Proxy data for north and east Africa show that the beginning of the Meghalayan period is clearly traced as a transition from humid to arid conditions, recorded about 4.2 thousand cal. years ago ^[15]. According to the core of the deposits of Lake Garba Guracha in Ethiopia, the transition to the arid conditions of the Meghalayan period is clearly traced in many parameters (total carbon content, values of ?13 C, C/N ratio, etc.). The thermal maximum is dated from 9 to 5.8 thousand cal. years ago, with a peak of warming of about 7 thousand cal. years ago, which correlates with data on lake cores from other lakes in Africa ^[16]. According to isotope-oxygen records of tree rings studied within the Tibetan plateau in China, it is shown that the sharp aridization of the climate in China began about 2 thousand years BC, i.e. about 4 thousand years ago, at the beginning of the Meghalayan Holocene period. It is shown that climate aridization in China began in the middle of the Holocene, and a rapid decrease in climate humidity was detected between 4000 and 3500 years ago, i.e. in this region the transition to arid conditions was not abrupt and rapid, but rather prolonged, for several centuries ^[17].

The Meghalayan period attracts special attention of researchers around the world, because there is evidence that this event, marking the transition to arid conditions, influenced the development of human communities on a global scale and led to global social changes, such as population migration, cultural change, etc. For example, droughts in China, which began about 4 thousand years ago. cal. years ago, they contributed to the change of Neolithic cultures and were probably the main cause of population migration and social transformations ^[17]. It is shown that droughts that began about 4.2 thousand cal. years ago, led to a sharp decline in the population in all Middle Eastern regions (except Cyprus), which caused a demographic crisis ^[18]. The consequences of the event 4.2 thousand cal years ago led to the fall of the Akkadian civilization in Mesopotamia, the possible collapse of urban communities in the southern Levant, the decline of well-known settlements in Southern Arabia and significant changes in settlement systems and irrigation technologies in Southern Iran; the drought conditions seriously affected non-irrigated agricultural systems in northern Mesopotamia ^[19].

Application of the scheme of the new three-term division of the Holocene for the Russian Arctic

The issues of Holocene periodization for the Arctic regions of the Russian cryolithozone were considered in the PhD thesis of N.A.Budantseva ^[20], devoted to the Holocene dynamics of the heaving mounds of the Bolshezemelskaya tundra and re-vein ice in the north of Western Siberia. The Holocene division was based on the scheme proposed in 1982 by Yu.K.Vasilchuk ^[6]. The time boundaries of the Holocene periods were clarified for the selected regions of the Russian Arctic – the north-east of the European part of Russia and the north of Western Siberia. The Holocene optimum, dated from 9.5 to 4.3 thousand years ago, was considered the longest and most significant event of the Holocene, the pre-optimal period was determined from 12 to 9.5 thousand years ago, the post-optimal period – from 4.3 thousand years ago to the present. It was found that the Holocene optimum period in the north of Western Siberia had an increased continental climate, when summer air temperatures were 2-3 °C higher than modern ones, and winter temperatures were close to the average temperatures of the last 60-70 years, or 1-3 °C lower. During this period, there was an active formation of peat bogs within thermokarst basins and syngenetic growth of re-vein ice in them. The postoptimal period of the Holocene as a whole is characterized by a noticeable decrease in the rate of peat accumulation or a complete cessation of peat accumulation, the desiccation of peat bogs led to an almost complete cessation of the growth of ice veins; during this period syngenetic re-vein ice grew mainly in accumulative areas of floodplains and laids. It should be noted that this division of the Holocene closely corresponds to the new Holocene periodization scheme, which suggests a three-term division into the Greenland, North Grippian and Meghalayan periods, their age boundaries and climatic trends correlate well, for example, the age of the lower boundary of the Meghalayan (post-optimal) period and the general tendency to decrease humidity and cooling of the climate in the Arctic regions of the north-east of the European parts of Russia and the north of Western Siberia.

To assess the severity of Holocene events for the territory of the Russian Arctic, especially changes in winter temperature parameters, data on the isotopic composition (values of ?18 ^O) in re-vein ice, the age of which was determined on the basis of radiocarbon dating of host sediments or microorganics extracted from vein ice, were analyzed. Reconstructions of the average January air temperature in the Holocene were performed based on the dependence of the values of ? ¹⁸ O in vein ice on T _cf.January, obtained by Yu.K.Vasilchuk ^{[21, 22]} and refined in recent years for some regions of the Russian cryolithozone (north of Western Siberia and the lower reaches of the Kolyma River).

It has been established that during most of the Holocene, syngenetic growth of re-vein ice occurred mainly in emerging peat bogs or swampy and blocked areas of marine and alluvial terraces. In the late Holocene, syngenetic growth of re-vein ice was often confined to accumulative areas of floodplains and laids. An analysis of a significant array of ¹⁴ C dates (about 200 definitions obtained by the author of the article and borrowed from other sources) for four regions of the Russian Arctic – the north of the European part of Russia, the north of Western Siberia, the lower reaches of the Kolyma River and the east of Chukotka – showed that the Greenland and North Grippian periods of the Holocene (between 11.7 and 4.2 thousand cal. years ago) were the periods of the most active development of peat bogs and re-vein ice. Early Holocene polygonal peat bogs (the formation of which occurred only during the Greenland Holocene period) were described in the area of Vorkuta, on the Yamal and Gydansky islands, on Bely Island, within the Alas in the lower reaches of the Kolyma River, in the east of Chukotka - on the Daurkin Peninsula and in the area of Anadyr. The most active polygonal-vein arrays were formed during the North Grippian period of the Holocene – between 8.2 and 4.2 thousand cal. years ago, the main part of the dating refers to this period. Holocene massifs with re-vein ice, dated to the North Grippian period, have been studied on the coast of the Baydaratskaya Bay, on Yamal, Gydan and Tazovsky islands, in the east of Chukotka. This period of time almost coincides with the previously established Holocene optimum, during which summer warming in the Russian Arctic reached a maximum, which led to a noticeable activation of thermokarst processes, waterlogging of thermokarst basins and alas and accumulation of peat bogs, within which intensive growth of re-vein ice occurred in winter.

The transition from the Northern Grippian to the Meghalayan Holocene period in the studied Arctic regions was characterized by a noticeable decrease in the rate of accumulation of peat bogs and/or the cessation of their formation, which is due to both a decrease in summer air temperatures (as indicated by the retreat of the forest boundary to the south, to its current position) and a decrease in the degree of waterlogging of territories due to the development of the drainage network. Polygonal peat bogs on the watershed surfaces were formed much less frequently, as evidenced by their few finds in the Russian Arctic – we have described massifs in the area of the village of Bovanenkovo on the Yamal Peninsula, in the area of the village ofChersky in the lower reaches of the Kolyma River and the village of Lavrentia in the east of Chukotka, for which ¹⁴ dates from 5.2 to 2.4 thousand cal were obtained. years ago. However, syngenetic growth of re-vein ice during this period occurred within the limits of young emerging relief elements – floodplains of rivers (for example, the Yerkutayakha River on the Yamal Peninsula, the Kolyma River and its channels), the laida of the Kara Sea off the coast of the Gydan Peninsula. The age of the veins formed here varies from 5 to 0.9 thousand cal. years ago.

Approximate reconstructions of the average January air temperature for the Russian Arctic. The reconstructions of the average January air temperature for four regions of the Russian Arctic performed by the author and prof. Yu.K.Vasilchuk showed that for most of the studied areas higher average January temperatures were obtained for the Greenland and North Grippian periods, and only for the east of Chukotka there was an increase in the average January temperature in the Meghalaya period.

For the north of the European part of Russia, it is shown that in the middle of the Greenland Holocene period (between 10.5 and 9-9.7 thousand cal. years ago) the average January air temperature varied between -23 and -25°C, during the North Grippian Holocene period (between 9 and 4 thousand cal. years ago) – from -24 to -28.5°C ^[23-25]. For the north of Western Siberia, it is shown that during the Greenland Holocene period (between 11.7 and 8.2 thousand cal. years ago), the average January air temperature varied between -21.3 and -27.7 °C, for Greenland – the first half of the Northern Grippian period (between 11.4 and 5.3 thousand cal. The average January air temperature varied from -22.3 to -27.8°C, and since the end of the Northern Grippian – during the Meghalayan period (between 5 and 0.9 thousand cal years ago), the average January air temperature varied between -24.1 and -27.8°C ^[26]. At the same time, there was a decrease in temperature values from the western coast of Yamal to the eastern and north-eastern regions of the Gydan Peninsula, which is typical for modern temperatures.

For the lower reaches of the Kolyma River, it was found that the average January air temperature during the Holocene varied in approximately the same range: from -40.7 to -33.8 °C during the Greenland period, from -38.6 to -33.3 °C during the Northern Grippian period and from -41.5 to -33 °C during the Meghalayan period. This most likely indicates the stability of winter climatic conditions in the north of Yakutia during the Holocene, determined by the influence of the Siberian anticyclone ^{[27, 28]}.

For the east of Chukotka, we obtained data that in the first half of the Greenland period (between 11.7 and 9 thousand cal. years ago) the average January air temperature varied from -23 to -27 ° C, during the Greenland – the first half of the Northern Grippian periods (between 11 and 5 thousand cal. years ago) the average January air temperature varied from -24 to -28.5 ° C. According to one massif with re-vein ice, dated by the host sediments to the Meghalayan period (from 4.5 to 2.5 thousand cal. years ago), data were obtained on the average January air temperature from -22.5 to -20 °C, which is noticeably higher than in previous Holocene periods ^{[29, 30, 31]}. 

            Holocene polygonal vein arrays as paleoarchives of winter temperature conditions in the Arctic. Studies of Holocene massifs with re-vein ice as paleoarchives of winter temperature conditions of the Holocene in the Arctic have been carried out in the last 10 years in the north of Western Siberia, in the Lena River delta, in the north and central part of Yakutia, in northern Alaska, northwestern Canada (Yukon).

T.Opel and co-authors ^[32] conducted detailed studies of Holocene polygonal vein arrays on the coast of the Dmitry Laptev Strait, in the outcrop of the Oygos Yar. Dating of organic matter from the Alas sediments containing veins showed that they accumulated during almost the entire Holocene – from the Greenland to the beginning of the Meghalayan periods (from 11.5 to 3.6 thousand cal. years ago). Dating of organic matter from ice of the most thoroughly tested vein showed that it was formed in the late Holocene, the youngest dating from 0.25 to 0.5 thousand years were obtained in the central part of the vein, the marginal parts of the vein were dated at 1.5 thousand years. A detailed isotope curve with a resolution of 2.5 cm was obtained from the vein . It is shown that the highest values of ?18 ^O (from -23 to -21 %) are located in the youngest parts of the vein, the lowest values of ?18 ^O (from -27 to -25 %) are characteristic of the most ancient fragments ^[33]. In this work, it is emphasized that the allocation of the warm period in the middle of the Holocene is not entirely correct, since the warmest winters are inherent in modernity.

            Holocene re-vein ices were investigated by X.Mayer and co-authors ^[34] in the Lena River delta. The age of the veins was determined by ¹⁴ With AMS dating of microorganics extracted from the ice, it was from 7.3 thousand cal. years to the present. A high-resolution isotope-oxygen record was obtained from the veins, a stable positive trend of ?18 ^O values over the past 7 thousand years from -26, -28, to -23, -24 was shown, which, according to the authors, indicates a trend of increasing winter air temperatures, especially noticeable in the second half of the Holocene – during the Northern Grippian and the Meghalayan period ^[34]. Subsequently, S. Vetterich and co-authors ^[35], conducting comprehensive studies of Holocene deposits of O.However, in the Lena River delta, it was shown that Holocene veins are characterized by rather low values of isotopic composition (the average values of ?18 ^O vary between -28 and -25%), which is not much lower than the average values for Late Pleistocene ice (the average values of ?18 ^O vary between -30 and -29%), therefore, it is incorrect to unambiguously to talk about the trend of increasing winter air temperatures in the Holocene in this region ^[35].

            For the lower reaches of the Kolyma River (district ofIt was shown by S. Vetterich and co-authors ^[36] that thermokarst processes were most active in the warmest phase of the Holocene, between 10.5 and 8 thousand years ago (i.e. during the Greenland stage), the stage of the Alas waterlogging was dated between 10.5 and 3.5 thousand years ago. In the Meghalayan period of the Holocene, after 3.5 thousand years ago, the growth of re-vein ice occurs within the forming peat bog, for which an average value of ?18 ^O -26.6 % was obtained, close to the values in the Holocene veins of other areas of the Nizhnekolymsky district ^[36]. According to G.Schwamborn and co-authors ^[37] in northern Yakutia (the Beenchime-Salaatinsky crater area), thermokarst activity peaked between 7.6 and 6.1 thousand cal. years ago (during the North Grippian period of the Holocene), during this period there was an active formation of thermokarst lakes. Between 5.7 and 1.5 thousand cal. years ago (at the end of the North Grippian – Meghalayan period) peat accumulation and the formation of re-vein ice began, the average value of ?18 ^O in the ice of one of the studied veins is -26% ^[37], which is close to the value for veins of this age studied in the lower reaches of the Kolyma River.

            For the north of Yakutia and Chukotka, it was found that summer conditions at the beginning of the Holocene (Greenland Holocene period) were noticeably warmer and wetter compared to modern ones. This contributed to the activation of thermokarst processes, the formation of lakes and swamps in the center of the polygons and the wide spread of peat bogs, which, due to the thermophysical properties of peat in winter, were areas of the most intensive formation of re-vein ice. For the Lower Kolyma lowland, it is shown that during the Holocene optimum, about 50% of the surface of the Edom strata were changed under the influence of thermokarst processes and turned into alasic depressions. In the late Holocene, about 11% of the edom complexes were transformed by alas. Currently, about 30% of the territory of the lowlands is occupied by lake basins, the development of which continues at the present time ^[38]. Radiocarbon dating (from 9.5 to 8 thousand years ago) of the remains of high-stemmed birches, large alder and willow bushes found in Holocene deposits of the Kolyma Lowland, islands of the Novosibirsk Archipelago and Northern Chukotka show that at the beginning of the Holocene (the second half of the Greenland period) forest vegetation grew much north of their modern range ^[39].

            In many Arctic regions of Alaska and Canada, the formation of re-vein ice at the beginning and middle of the Holocene was probably very limited due to deep thawing and the development of thermokarst. In Alaska, the resumption of the growth of ice veins dates back no earlier than 5 thousand years ago. In Alaska X .Mayer with co - authors and M.Kanevsky and co-authors ^{[40, 41, 42]} investigated Late Holocene veins in the Volt Creek Tunnel and in the Prudhoe Bay and Barrow areas. ¹⁴ From the dating of the wood remains from the deposits containing the veins in the Wattle Creek tunnel (2500 and 3445 years) showed that the veins were formed during the Meghalayan period, 3-2.5 thousand years ago.

            M.Grinter and co-authors ^[43] conducted studies of Holocene veins on the Blackstone Plateau, central Yukon, whose age was determined from 6.4 to 0.9 thousand cal. years. According to the authors, in the first half of the Middle Holocene (11.7-6.5 thousand cal. years ago – Greenland – the first half of the Northern Grippian period of the Holocene) ice veins were not formed due to the predominance of warm and humid climatic conditions: summer air temperature was 2-7 ° C higher, precipitation 1.5-2 times higher than today, which contributed to the strengthening of thermokarst, an increase in the power of the active layer in Alaska and in the western part of the Arctic Canada and partial degradation of previously formed ice veins. The growth of ice veins resumed about 6.5 thousand cal. years ago, with a peak of activity between 4 and 0.9 thousand cal. years ago, i.e. during the Meghalayan period, which is confirmed by data from other regions of Arctic Canada and Alaska ^[43].

            Studies of the development of polygonal landscapes were carried out by J.Walter and co-authors ^[44] on the Yukon Coastal Plain in Canada. According to their findings, in the middle of the Holocene (7-6 thousand cal. years ago – the Northern Grippian period, Holocene optimum) conditions of strong waterlogging prevailed, polygonal-vein arrays were formed in subaqual conditions. Then followed a break in the development of arrays, lasting about 5-6 thousand years. The resumption of the growth of re-vein ice is dated to the last millennium. According to the authors, the main factors in the development of polygonal arrays were changes in geomorphological and hydrological conditions, the connection with climatic fluctuations was not traced. The only extreme climatic factor was the Holocene thermal optimum, during which the accumulation of peat bogs began in vast areas of the Yukon coastal plain ^[44].

Conclusion

According to the decision of the International Commission on Stratigraphy (IUGS) in 2018, the Holocene was divided into three periods: Greenland (from 11,700 to 8,200 cal. years ago), North Grippian (from 8200 to 4200 cal. years ago) and Meghalayan (beginning 4200 cal. years ago). The new division of the Holocene was based on a natural, usually short-term episode recorded in climate proxy data (i.e., changes in the oxygen isotope composition in glacial cores and speleothems), but which, nevertheless, led to climate changes on a global scale: the beginning of the Greenland period, as a transition from the late Pleistocene to Holocene, records the transition to warmer climatic conditions, “event 8.2 thousand years ago” records a short–term cooling of the climate and “event 4.2 thousand years ago” - the transition to arid conditions. The last event caused not only changes in the natural environment, but also had a significant impact on the human civilizations that existed at that time. 

For the polar and subpolar regions of Russia, the Holocene division scheme proposed by N.A.Khotinsky (1977) was widely used, according to which the lower boundary of the Holocene dates back to 10500-10300 years, while 5 periods (preboreal, boreal, Atlantic, subboreal and subatlantic) were distinguished within the Holocene. In 1982, for the Arctic regions of the Russian cryolithozone, Y.K.Vasilchuk proposed a three–term division of the Holocene in 1982, within which three stages were distinguished - pre-optimal, optimal and post-optimal. The time limits of the Holocene optimum were determined from 9 to 4.5 thousand years ago, it was considered as a period of increase in average annual air temperatures, but at the same time winter conditions during the optimum period were not milder than modern ones, and in some periods of winter were colder, which contributed to the intensive growth of syngenetic re-vein ice.

For four key regions of the Russian cryolithozone – the north of the European part of Russia, the north of Western Siberia, the lower reaches of the Kolyma River and the east of Chukotka – it is shown that, taking into account the new periodization of the Holocene, the Greenland and North Grippian periods of the Holocene (between 11.7 and 4.2 thousand years ago) were the time of the most active development of peat bogs and the simultaneous formation of re-vein ice in them. The Meghalayan Holocene period was characterized by a marked decrease in the development of peat bogs, but syngenetic growth of re-vein ice continued within the emerging floodplains and laids, especially within the torn-off areas. The reconstructed average January air temperatures for four key regions of the Russian cryolithozone showed that the Greenland and North Grippian periods of the Holocene were characterized by slightly higher values (on average 1-2 Oc higher) than the Meghalayan, with the exception of eastern Chukotka, for which an increase in the average January air temperature during the Meghalayan period was noted.

In contrast to the Russian cryolithozone, for many Arctic regions of Alaska and Canada, the formation of re-vein ice at the beginning and middle of the Holocene (up to the middle of the Northern Grippian period) was probably very limited due to deep thawing and the development of thermokarst. In Alaska, the resumption of the growth of ice veins dates back no earlier than 5 thousand years ago, in the north of Canada – about 6.5 thousand years ago, while in many areas during the Meghalayan period, active formation of re-vein ice was noted.